1. Field of the Invention
The present invention is directed to improved can tooling components for the fabrication of metal containers, and in particular, whisker-reinforced silicon nitride can tooling components for the fabrication of drawn and ironed aluminum can bodies.
2. Description of Related Art
A two-piece aluminum beverage can has a top lid component and a body having an integrally formed closed end. The can is typically made by blanking circular disks from an aluminum sheet and forming a cup from the disk by placing the disk in a cup-forming die and moving a cup-forming punch through the cup-forming die. The cup is transferred to a body-forming apparatus where it is forced through a body-forming die by a body-forming punch. The body-forming die includes a successive plurality of rings known as the redraw and ironing rings. The clearance between the body-forming punch and the plurality of rings becomes progressively smaller as the cup moves through the die, so that the cup walls are "ironed" out into a thin section. A doming die then presses the bottom of the can body into a concave configuration for added strength.
After the can body is formed, the open end of the can is trimmed to the desired length. The can may then be washed, dried, and then necked on the open end. For cans that are to be printed with a label, the can may be transferred to a printer before necking. After printing, the can is dried in a drying oven.
A typical can body-forming apparatus is capable of producing about 240,000 cans per day based on 24-hour operation. Over 83 billion aluminum beverage cans are produced in the United States every year.
Because of the tremendous volume of aluminum beverage cans manufactured each year, any slight improvement in the efficiency of the manufacturing process can result in tremendous savings to the manufacturer. Over the years, for instance, the industry has made significant efforts to reduce the weight of the aluminum cans in order to reduce material costs. As a result, the weight of aluminum beverage cans has been significantly reduced with simultaneous improvements in strength, dimensional consistency, and quality of finish. However, further improvements are still sought.
The can tooling components, such as the body-forming punch, the redraw and ironing rings and the doming die, must be sufficiently strong and abrasion resistant to consistently produce acceptable cans. Traditionally, steel was used as the material for the body-forming punch. Recently, steel has been replaced in some applications by tungsten carbide (WC). However, tungsten carbide has a number of disadvantages. For example, tungsten carbide is extremely heavy, having a density of about 15 g/cm.sup.3.
U.S. Pat. No. 5,095,730 by Lauder and issued on Mar. 17, 1992, discloses whisker-reinforced ceramic tools and components, specifically components used in the manufacture of two-piece aluminum beverage cans.
This patent discloses a variety of whisker-reinforced matrix materials, including alumina, silicon nitride, silicon carbide, zirconia, boron carbide and titanium diboride. This patent application also teaches that 2 to 40 weight percent of whiskers in the matrix are preferred, the whiskers having a diameter of from 0.35 to 0.65 micrometers. The application also states that whisker-reinforced alumina is the most preferred material for manufacturing two-piece body cans. It is disclosed that the whisker-reinforced materials impart certain advantages including reduced friction, reduced alumina formation on the formed cans, reduced scoring on the inside and outside of cans, longer useful service life of the tool components, lighter weight and ease of grinding.
However, most whisker-reinforced materials, including whisker-reinforced alumina, have a number of disadvantages,. particularly when used as a body-forming punch. For example, the surface of a punch fabricated from whisker-reinforced alumina having about ten volume percent silicon carbide whiskers rapidly shows evidence of surface fatigue and mechanical strength degradation when used to manufacture aluminum can bodies. Similar results are obtained with an alumina body-forming punch having about 15 volume percent silicon carbide whiskers. This surface fatigue significantly reduces the surface friction of the body-forming punch so that the punch is no longer able to form an acceptable can body. Further, the mechanical strength is degraded such that the strength is not high enough to prevent cracks from forming in the body-forming punch. Therefore, the punch is unable to produce an acceptable number of can bodies.
Further, the use of whisker-reinforced alumina and other materials that the present inventors are aware of requires that the can body be produced with a trim height variance. For example, the trim height on an aluminum can body is typically between about 0.094 inch and about 0.25 inch (2.4 mm to 6.4 mm). The trim height is the excess amount of aluminum remaining on the top of the can body after the can body is formed in the body-forming apparatus. The trim height results from an excess amount of aluminum intentionally put in the can body to account for deviations that occur in virgin aluminum can sheet thickness and can wall thickness, particularly when the body-forming apparatus is in the startup, or warmup, stage. During the startup stage, when the body-forming punch is cold, the punch has a smaller diameter than when the punch is warm and therefore does not iron the can body to its maximum height. After the punch has formed a number of can bodies, the punch warms and increases in diameter. The can bodies formed thereafter have a reduced can wall thickness and an excess of aluminum on the top edge as a result. The difference in trim height between the first cans and the subsequently produced cans is known as the trim height variation. The trim height variation must be accounted for since a punch apparatus can shut down and restart many times each day in a typical can-making operation.
If the trim height variation was substantially minimized or eliminated, the total volume of aluminum per can could advantageously be decreased. A reduction of the starting gauge of the aluminum by about 0.0001 inch (0.003 mm) will advantageously result in about $1 million per year savings for the typical aluminum can manufacturer who produces about 4.6 billion cans annually.
It would therefore be advantageous to provide a body-forming tool material that is less susceptible to thermal expansion, surface fatigue and failure than punch tools heretofore known. Further, it would be advantageous to provide a body-forming punch wherein the trim height variance on the aluminum cans due to thermal expansion of the punch is reduced or eliminated.